In the development of catalytic converters for removing hydrocarbons, carbon monoxide, and nitrogen oxides from engine exhaust gases, a common approach has been to provide a two-stage system. In the first stage the raw exhaust gases are passed under net reducing conditions over a suitable catalyst in the absence of added air to effect reduction of nitrogen oxides (NO.sub.x). At least a stoichiometric proportion of air is then added to the gases and the resulting mixture is passed through the second stage to effect oxidation of remaining CO and hydrocarbons. Superior catalysts for the NO.sub.x reduction stage comprise rhodium or rhodium-nickel combinations, while palladium and/or platinum are preferred in the oxidation stage.
Although the two-stage system is effective, certain disadvantages have come to be recognized. Firstly, most NO.sub.x reduction catalysts become relatively ineffective under net oxidizing conditions, i.e., when more than stoichiometric amounts of oxygen are present. It has therefore been considered necessary to provide relatively rich air/fuel (A/F) mixtures, with resultant loss in fuel economy. Secondly, air injection for the second stage oxidation reactions tends to oxidize sulfur compounds in the gases to SO.sub.3, a more noxious pollutant than SO.sub.2. Finally, the provision of air injection and means of control therefor presents a troublesome additional expense. Considerations such as these have led to attempts to develop a single stage "three-way conversion" (TWC) system in which adequate conversion of all three types of pollutants could be obtained simultaneously without air addition, while operating with A/F ratios controlled to vary only moderately from stoichiometric (which is ordinarily about 15 lbs. of air per lb. of gasline). A successful TWC operation should give at least about 70% conversion of NO.sub.x, CO, and hydrocarbons over a range of A/F ratios varying by plus or minus 0.1 units from stoichiometric. Achieving and maintaining this type of performance over a substantial period of time is critically dependent upon the nature of the catalyst or catalysts employed.
The present invention is based upon my discovery of a type of catalyst composite which appears to achieve the foregoing objectives to a greater extent than other presently known catalysts. This catalyst comprises at least two physically separate but contiguously arranged components. The first component comprises a porous inert support upon which is dispersed a minor proportion of nickel and/or cobalt and a much smaller proportion of rhodium. The second component comprises an inert porous support upon which is dispersed a minor proportion of platinum and/or palladium, and preferably nickel and/or cobalt. The most critical aspect of the invention resides in maintaining an essentially complete physical separation between the rhodium component and the platinum and/or palladium component. It has been found that when all the metal components are uniformly distributed on the same support, as by successive impregnations or co-impregnation, the resulting catalyst displays an initially satisfactory TWC performance, but with aging rapidly loses activity for NO.sub.x conversion. This would appear to indicate that deactivation of the rhodium component takes place, and it is speculated that some type of alloying of the rhodium with the platinum and/or palladium may be involved. This would be particularly critical in view of the fact that the proportion of rhodium normally utilized is only about 1/10 to 1/100 of the proportion of platinum and palladium utilized. In any event it has been found that a much more stable TWC operation can be achieved by impregnating the rhodium upon an entirely separate support than the support on which the platinum and/or palladium component is deposited. Several modes of achieving this physical separation of components will be described hereinafter.